sched, latencytop: incorporate review feedback from Andrew Morton

Andrew had some suggestions for the latencytop file; this patch takes care
of most of these:

* Add documentation
* Turn account_scheduler_latency into an inline function
* Don't report negative values to userspace
* Make the file operations struct const
* Fix a few checkpatch.pl warnings

Signed-off-by: Arjan van de Ven <arjan@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 71 insertions, 12 deletions

diff --git a/kernel/latencytop.c b/kernel/latencytop.c
index 449db466bdbc..ca07c5c0c914 100644
--- a/kernel/latencytop.c
+++ b/kernel/latencytop.c
@@ -9,6 +9,44 @@
  * as published by the Free Software Foundation; version 2
  * of the License.
  */
+
+/*
+ * CONFIG_LATENCYTOP enables a kernel latency tracking infrastructure that is
+ * used by the "latencytop" userspace tool. The latency that is tracked is not
+ * the 'traditional' interrupt latency (which is primarily caused by something
+ * else consuming CPU), but instead, it is the latency an application encounters
+ * because the kernel sleeps on its behalf for various reasons.
+ *
+ * This code tracks 2 levels of statistics:
+ * 1) System level latency
+ * 2) Per process latency
+ *
+ * The latency is stored in fixed sized data structures in an accumulated form;
+ * if the "same" latency cause is hit twice, this will be tracked as one entry
+ * in the data structure. Both the count, total accumulated latency and maximum
+ * latency are tracked in this data structure. When the fixed size structure is
+ * full, no new causes are tracked until the buffer is flushed by writing to
+ * the /proc file; the userspace tool does this on a regular basis.
+ *
+ * A latency cause is identified by a stringified backtrace at the point that
+ * the scheduler gets invoked. The userland tool will use this string to
+ * identify the cause of the latency in human readable form.
+ *
+ * The information is exported via /proc/latency_stats and /proc/<pid>/latency.
+ * These files look like this:
+ *
+ * Latency Top version : v0.1
+ * 70 59433 4897 i915_irq_wait drm_ioctl vfs_ioctl do_vfs_ioctl sys_ioctl
+ * |    |    |    |
+ * |    |    |    +----> the stringified backtrace
+ * |    |    +---------> The maximum latency for this entry in microseconds
+ * |    +--------------> The accumulated latency for this entry (microseconds)
+ * +-------------------> The number of times this entry is hit
+ *
+ * (note: the average latency is the accumulated latency divided by the number
+ * of times)
+ */
+
 #include <linux/latencytop.h>
 #include <linux/kallsyms.h>
 #include <linux/seq_file.h>
@@ -72,7 +110,7 @@ account_global_scheduler_latency(struct task_struct *tsk, struct latency_record
                                 firstnonnull = i;
                         continue;
                 }
-                for (q = 0 ; q < LT_BACKTRACEDEPTH ; q++) {
+                for (q = 0; q < LT_BACKTRACEDEPTH; q++) {
                         unsigned long record = lat->backtrace[q];
 
                         if (latency_record[i].backtrace[q] != record) {
@@ -101,31 +139,52 @@ account_global_scheduler_latency(struct task_struct *tsk, struct latency_record
         memcpy(&latency_record[i], lat, sizeof(struct latency_record));
 }
 
-static inline void store_stacktrace(struct task_struct *tsk, struct latency_record *lat)
+/*
+ * Iterator to store a backtrace into a latency record entry
+ */
+static inline void store_stacktrace(struct task_struct *tsk,
+                                        struct latency_record *lat)
 {
         struct stack_trace trace;
 
         memset(&trace, 0, sizeof(trace));
         trace.max_entries = LT_BACKTRACEDEPTH;
         trace.entries = &lat->backtrace[0];
-        trace.skip = 0;
         save_stack_trace_tsk(tsk, &trace);
 }
 
+/**
+ * __account_scheduler_latency - record an occured latency
+ * @tsk - the task struct of the task hitting the latency
+ * @usecs - the duration of the latency in microseconds
+ * @inter - 1 if the sleep was interruptible, 0 if uninterruptible
+ *
+ * This function is the main entry point for recording latency entries
+ * as called by the scheduler.
+ *
+ * This function has a few special cases to deal with normal 'non-latency'
+ * sleeps: specifically, interruptible sleep longer than 5 msec is skipped
+ * since this usually is caused by waiting for events via select() and co.
+ *
+ * Negative latencies (caused by time going backwards) are also explicitly
+ * skipped.
+ */
 void __sched
-account_scheduler_latency(struct task_struct *tsk, int usecs, int inter)
+__account_scheduler_latency(struct task_struct *tsk, int usecs, int inter)
 {
         unsigned long flags;
         int i, q;
         struct latency_record lat;
 
-        if (!latencytop_enabled)
-                return;
-
         /* Long interruptible waits are generally user requested... */
         if (inter && usecs > 5000)
                 return;
 
+        /* Negative sleeps are time going backwards */
+        /* Zero-time sleeps are non-interesting */
+        if (usecs <= 0)
+                return;
+
         memset(&lat, 0, sizeof(lat));
         lat.count = 1;
         lat.time = usecs;
@@ -143,12 +202,12 @@ account_scheduler_latency(struct task_struct *tsk, int usecs, int inter)
         if (tsk->latency_record_count >= LT_SAVECOUNT)
                 goto out_unlock;
 
-        for (i = 0; i < LT_SAVECOUNT ; i++) {
+        for (i = 0; i < LT_SAVECOUNT; i++) {
                 struct latency_record *mylat;
                 int same = 1;
 
                 mylat = &tsk->latency_record[i];
-                for (q = 0 ; q < LT_BACKTRACEDEPTH ; q++) {
+                for (q = 0; q < LT_BACKTRACEDEPTH; q++) {
                         unsigned long record = lat.backtrace[q];
 
                         if (mylat->backtrace[q] != record) {
@@ -186,7 +245,7 @@ static int lstats_show(struct seq_file *m, void *v)
         for (i = 0; i < MAXLR; i++) {
                 if (latency_record[i].backtrace[0]) {
                         int q;
-                        seq_printf(m, "%i %li %li ",
+                        seq_printf(m, "%i %lu %lu ",
                                 latency_record[i].count,
                                 latency_record[i].time,
                                 latency_record[i].max);
@@ -223,7 +282,7 @@ static int lstats_open(struct inode *inode, struct file *filp)
         return single_open(filp, lstats_show, NULL);
 }
 
-static struct file_operations lstats_fops = {
+static const struct file_operations lstats_fops = {
         .open           = lstats_open,
         .read           = seq_read,
         .write          = lstats_write,
@@ -236,4 +295,4 @@ static int __init init_lstats_procfs(void)
         proc_create("latency_stats", 0644, NULL, &lstats_fops);
         return 0;
 }
-__initcall(init_lstats_procfs);
+device_initcall(init_lstats_procfs);